It is conventional for a light unit, e.g. a rear light unit for a vehicle, to comprise a plurality of lights (rear light, brake light, indicator light, reversing light, and rear fog light) with the lamps of all the lights being connected to a common ground and with each of them being powered via a respective conductor extending from the power terminal of the lamp to a special switch situated on the dashboard.
Thus, in order to power its various lamps, a five-light unit requires five specific power conductors, each of sufficient section to convey power currents that may typically reach two amps.
A multiplexed control system has recently been developed for a group of lights in which a single power supply line goes to the light unit, and a low current control line conveys signals containing instructions for turning each of the lights on and off. A specialized decoding or demultiplexing electronic circuit serves to recognize the instructions contained in the signals and to control switch means accordingly to switch the lamps in each of the lights on or off selectively. The switch means may be relays, but they are preferably power semiconductors.
In the prior art, the decoding module and the power semiconductors are integrated in a single assembly which may either be incorporated in the light unit, or else may be in the form of a separate unit situated in the vehicle close to said light unit. A considerable drawback of this prior solution lies in the practical impossibility of making a single standard assembly for various different types of vehicle light unit. For example, there is considerable variation between the number of lights in a front or rear light unit and the functions they are to perform between a bottom-of-range vehicle and a top-of-range vehicle.
In addition, the decoding module must be specially designed to be capable of withstanding the severe temperature environment caused by heat being dissipated in the power switches.